mcq-network-pftables.rst

Port forwarding tables

task_id:	networkpftable

Port forwarding tables are used in tree-shaped networks to automatically build the forwarding tables that allow the network nodes to forward packets towards their destinations.

.. question:: portforwarding0
  :nb_prop: 3
  :nb_pos: 2

  1. Consider the tree-shaped network shown in the figure below. Assume that the port-forwarding tables are as shown in the figure.

  .. tikz::
     :libs: positioning, matrix, arrows

     \tikzstyle{arrow} = [thick,->,>=stealth]
     \tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
     \tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
     \tikzset{ftable/.style={rectangle, dashed, draw} }
     \node[host] (A) {A};
     \node[router, right=of A] (R1) { R1 };
     \node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l}
     Dest. & OutPort  \\
     \hline
     A & W \\
     \end{tabular}};
     \node[router,right=of R1] (R2) {R2};
     \node[ftable, right=of R2] (FR2) { \begin{tabular}{l|l}
     Dest. & OutPort \\
     \hline
     A & W \\
     \end{tabular}\\};
     \node[router,below=of R1] (R3) {R3};
     \node[ftable, below=of R3] (FR3) { \begin{tabular}{l|l}
     Dest. & OutPort \\
     \hline
     A & N  \\
     \end{tabular}\\};
     \node[host, right=of R3] (B) {B};
     \node[host, left=of R3] (C) {C};

     \path[draw,thick]
     (A) edge (R1)
     (R1) edge (R2)
     (R1) edge (R3)
     (R3) edge (B)
     (C) edge (R3);

     \draw[arrow, dashed] (FR1) -- (R1);
     \draw[arrow, dashed] (FR2) -- (R2);
     \draw[arrow, dashed] (FR3) -- (R3);


  Which of the following affirmations are true ?

  .. positive:: If `C` sends a packet towards `A`, `R3` will immediately forward it on its `North` interface

  .. positive:: If `B` sends a packet towards `C`, `R3` will immediately forward it on its `West` interface

  .. positive:: If `A` sends a packet towards `C`, `R1` will send it to both its `East` and `South` interfaces.

  .. positive:: If `A` sends a packet towards `B`, `R1` will send it to both its `East` and `South` interfaces.

  .. negative:: If `A` sends a packet towards `B`, `R1` will send it only to its `East` interface.

     .. comment:: Since `R1` does not know how to reach destination `B`, it must send the packets on all interfaces except the one from which it has been received.


  .. negative:: If `A` sends a packet towards `C`, `R1` will send it only to its `South` interface.

     .. comment:: Since `R1` does not know how to reach destination `C`, it must send the packets on all interfaces except the one from which it has been received.

  .. negative:: If `R1` receives from `R3` a packet whose destination is `A`, it sends it on both its `West` and `East` interfaces.

.. question:: portforwarding1
  :nb_prop: 3
  :nb_pos: 2

  2. Consider the tree-shaped network shown in the figure below. Assume that `A` has sent a packet towards `B` but `B` has not yet replied.

  .. tikz::
     :libs: positioning, matrix, arrows

     \tikzstyle{arrow} = [thick,->,>=stealth]
     \tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
     \tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
     \tikzset{ftable/.style={rectangle, dashed, draw} }
     \node[host] (A) {A};
     \node[router, right=of A] (R1) { R1 };
     \node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l}
     Dest. & OutPort  \\
     \hline
     A & W \\
     \end{tabular}};
     \node[router,right=of R1] (R2) {R2};
     \node[ftable, right=of R2] (FR2) { \begin{tabular}{l|l}
     Dest. & OutPort \\
     \hline
     A & W \\
     \end{tabular}\\};
     \node[router,below=of R1] (R3) {R3};
     \node[ftable, below=of R3] (FR3) { \begin{tabular}{l|l}
     Dest. & OutPort \\
     \hline
     A & N  \\
     \end{tabular}\\};
     \node[host, below=of R2] (B) {B};


     \path[draw,thick]
     (A) edge (R1)
     (R1) edge (R2)
     (R2) edge (R3)
     (R2) edge (B);

     \draw[arrow, dashed] (FR1) -- (R1);
     \draw[arrow, dashed] (FR2) -- (R2);
     \draw[arrow, dashed] (FR3) -- (R3);


     `B` sends a reply to `A`. Which of the following affirmations are true ?

  .. positive::

     Upon reception of this packet, the port forwarding table of `R3` will be updated as :

     ======  ========
     Dest.   OutPort
     ======  ========
     A       N
     B       E
     ======  ========


  .. positive::

     Upon reception of this packet, the port forwarding table of `R1` will be updated as :

     ======  ========
     Dest.   OutPort
     ======  ========
     A       W
     B       S
     ======  ========

  .. negative::

     Upon reception of this packet, the port forwarding table of `R2` will be updated as :

     ======  ========
     Dest.   OutPort
     ======  ========
     A       W
     B       W
     ======  ========

     .. comment:: When `R1` receives the packet sent by `B` towards `A`, it forwards it directly to `A`. This implies that `R2` will not receive this packet and thus cannot update its port forwarding table.

  .. positive::

     The port forwarding table of `R2` will never be updated with information about destination `B`.

  .. negative::

     Upon reception of this packet, the port forwarding table of `R3` will be updated as :

     ======  ========
     Dest.   OutPort
     ======  ========
     A       N
     B       N
     ======  ========